Optical single-bit recording and fluorescent readout utilizing aluminum oxide single crystals

ABSTRACT

The present invention provides methods and apparatuses for writing information to, reading information from, and erasing information on a luminescent data storage medium comprising Al 2 O 3 . The method includes writing and erasing of the information using photo ionization via sequential two-photon absorption and non-destructive reading the information using fluorescent detection. The apparatuses for writing and reading the information incorporate confocal detection and spherical aberration correction for multilayer volumetric fluorescent data storage. The methods also allow multilevel recording and readout of information for increased storage capacity.

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application is a continuation-in-part of U.S. applicationSer. No. 10/309,021, filed Dec. 4, 2002, entitled, “Aluminum OxideMaterial for Optical Data Storage,” which claims the priority of U.S.Provisional App. No. 60/336,749, filed Dec. 4, 2001, now abandoned, andU.S. Application Ser. No. 10/309,179, filed Dec. 4, 2002, entitled,“Method for Forming Aluminum Oxide Material Used in Optical DataStorage,” which claims the priority of U.S. Provisional App. No.60/417,153, filed Oct. 10, 2002. The entire disclosures and contents ofthe above applications are hereby incorporated by reference.

BACKGROUND OF THE INVENTION

[0002] 1. Field of the Invention

[0003] The present invention relates to methods of writing to, readingfrom and erasing information on a data storage medium.

[0004] 2. Description of the Prior Art

[0005] Various attempts have been made to design higher density datastorage media for computer devices to replace conventional storage mediasuch as magnetic disks, CD-ROMs, DVDs, etc. Many of the obstacles facedwith respect to developing improved data storage methods have beenassociated with inadequate storage material properties. For example,photopolymers have been investigated for use in one-bit or holographicdata storage. However, photopolymers exhibit strong dimensionalshrinkage. Also, most photo-sensitive polymers may be used only as WORMmedia (write once, read many times) and the rewritable photopolymers arestill unstable and show significant fatigue when write-read cycles arerepeated many times. Even write-once fluorescent photopolymers showstrong reduction of fluorescent output signals when read repeatedly. Anadditional problem with most photopolymers, as well as forphotorefractive crystals, another potential material for volumetricone-bit recording, is the necessity of using a femto-second high peakpower Ti-sapphire laser to achieve efficient two-photon absorption. Thistype of laser is big, expensive and suitable only for laboratorydemonstration.

[0006] Therefore, there exists a need for better materials for makinghigh density data storage devices.

SUMMARY OF THE INVENTION

[0007] It is therefore an object of the present invention to provide adata storage method utilizing an aluminum oxide material that allows forfast electronic processing in comparison with phase change transition orphoto-induced polymerization techniques for data storage.

[0008] It is a further object of the present invention to provide a datastorage method utilizing an aluminum oxide material that is capable ofachieving a write/read rate up to 100 Mbit per second.

[0009] It is yet another object of the present invention to provide adata storage method utilizing an aluminum oxide material that providesthe ability to perform parallel processing of multiple marks on a datastorage medium for an increase of write/read rate.

[0010] It is yet another object of the present invention to provide adata storage method utilizing an aluminum oxide material that provideshigh data storage density restricted only by diffraction limit and NA ofthe optical components.

[0011] It is yet another object of the present invention to provide adata storage method utilizing aluminum oxide material that provides thepossibility of multilevel data storage due to dependency of fluorescenceresponse from writing laser energy.

[0012] It is yet another object of the present invention to provide adata storage method utilizing aluminum oxide material that only requireslow laser light energies for writing and reading of information (pJ andnJ range).

[0013] It is yet another object of the present invention to provide adata storage method utilizing aluminum oxide material that providesextremely high temperature and time stability of stored data.

[0014] It is yet another object of the present invention to provide adata storage method utilizing aluminum oxide material that provides nodegradation of material performance after millions of write/read cycles.

[0015] According to a first broad aspect of the present invention, thereis provided a method of writing information to a data storage mediumcomprising the steps of: providing a luminescent data storage mediumcomprising Al₂O₃; and writing the information to the luminescent datastorage medium with an optical source.

[0016] According to a second broad aspect of the present invention,there is provided a method of reading information stored on a datastorage medium comprising the steps of: exciting a luminescent datastorage medium with an optical source to thereby cause the luminescentdata storage medium to emit a fluorescent light signal, wherein theluminescent data storage medium comprises Al₂O₃ and wherein the opticalsource emits a read laser beam having a wavelength in the range of anabsorption band of the luminescent data storage medium; and measuringthe laser induced fluorescence light signal from the luminescent datastorage medium, to thereby read the information stored on theluminescent data storage medium.

[0017] According to a third broad aspect of the present invention, thereis provided a method of erasing information stored on a data storagemedium comprising the steps of: providing a luminescent data storagemedium comprising Al₂O₃, the luminescent data storage medium having theinformation stored thereon; and illuminating the luminescent datastorage medium with an optical source to thereby erase the information.

[0018] According to a fourth broad aspect of the present invention,there is provided a method of writing information to a data storagemedium comprising the steps of: providing a luminescent data storagemedium comprising Al₂O₃; and writing the information to the luminescentdata storage medium by using a two-photon absorption technique and aphoto-ionization technique resulting in removal of an electron from acolor center in the luminescent data storage medium and moving theelectron to a thermally stable trap in the luminescent data storagemedium.

[0019] According to a fifth broad aspect of the present invention, thereis provided a method of reading information from a data storage mediumcomprising the steps of: exciting a luminescent data storage medium withan optical source having a wavelength in the range of an absorption bandof the luminescent data storage medium to thereby cause the luminescentdata storage medium to emit a fluorescent light signal, wherein theluminescent data storage comprises Al₂O₃ and color centers; andmeasuring the induced fluorescence signal from the luminescent datastorage medium, to thereby read the information stored on theluminescent data storage medium, wherein the method is performed in acondition of one-photon absorption without photo-ionization of the colorcenters and results in excitation of the color centers by the opticalsource to thereby cause the color centers to emit the fluorescencesignal.

[0020] According to a sixth broad aspect of the present invention, thereis provided a method of erasing information stored in the data storagemedium comprising steps of: providing a luminescent data storage mediumcomprising Al₂O₃, the luminescent data storage medium having theinformation stored thereon; and illuminating the luminescent datastorage medium with an optical source in conditions of two-photonabsorption to thereby erase the information.

[0021] According to a seventh broad aspect of the present invention,there is provided a apparatus comprising: a luminescent data storagemedium comprising Al₂O₃; and an optical source for writing informationto the luminescent data storage medium.

[0022] According to an eighth broad aspect of the present invention,there is provided a apparatus comprising: a luminescent data storagemedium comprising Al₂O₃; a first optical source for exciting theluminescent data storage medium to thereby cause the luminescent datastorage medium to emit a fluorescent light signal when information isstored on the luminescent data storage medium; and means for measuringthe emitted fluorescent light signal.

[0023] According to a ninth broad aspect of the present invention, thereis provided a apparatus comprising: a luminescent data storage mediumcomprising Al₂O₃; and writing means for writing information to theluminescent data storage medium by using a two-photon absorptiontechnique and a photo-ionization technique resulting in removal of anelectron from a color center in the luminescent data storage medium andmoving the electron to a thermally stable trap in the luminescent datastorage medium, the writing means comprising a first optical source.

[0024] Other objects and features of the present invention will beapparent from the following detailed description of the preferredembodiment.

BRIEF DESCRIPTION OF THE DRAWINGS

[0025] The invention will be described in conjunction with theaccompanying drawings, in which:

[0026]FIG. 1 is a schematic diagram illustrating writing information toa data storage medium of the present invention using a two-photonabsorption and reading information from a data storage medium of thepresent invention using laser-induced fluorescence with a confocaldetection scheme;

[0027]FIG. 2 shows optical absorption spectra of two crystals: a knownAl₂O₃:C crystal used in radiation dosimetry and an Al₂O₃:C,Mg singlecrystal according to a preferred embodiment of the present inventionwith a higher concentration of F⁺-centers (absorption at 255 nm) and newabsorption bands corresponding to F₂ ⁺ (2 Mg)— and F₂ ²⁺(2 Mg)-centersclearly distinguishing a new material;

[0028]FIG. 3 is a graph showing the absorption, excitation and emissionspectra of F₂ ²⁺(2 Mg)-centers created in Al₂O₃:C,Mg in an as-receivedor erased state, with an excitation spectrum of 520 nm emission bandcoinciding well with the absorption band at 435 nm assigned to the samedefect, and showing the wavelength dependence of a two-photonphoto-ionization cross-section for F₂ ²⁺(2 Mg)-centers;

[0029]FIG. 4 is a graph showing a continuous readout of fluorescentsignal at a 20 MHz repetition rate with the lifetime of F₂ ²⁺(2Mg)-center emission at 520 nm equal to 9±3 ns for an Al₂O₃:C,Mgluminescent material according to a preferred embodiment of the presentinvention;

[0030]FIG. 5 is a graph illustrating the multilevel data storageprinciple based on the proportionality between 435 and 335 nm absorptionband intensity and the “write” time for a Al₂O₃:C,Mg luminescentmaterial of a preferred embodiment of the present invention;

[0031]FIG. 6 is a graph showing the excitation and emission spectra ofF₂ ⁺(2Mg)-centers obtained as a result of photo-conversion of an F₂ ²⁺(2Mg)-center (“write” operation) in Al₂O₃:C,Mg crystals using pulsed bluelaser light with a wavelength of 430 nm;

[0032]FIG. 7 is a graph showing the lifetime measurement of F₂ ⁺(2Mg)-center emission at 750 nm equal to 80±5 ns for an Al₂O₃:C,Mgluminescent material according to a preferred embodiment of the presentinvention;

[0033]FIG. 8 is a graph showing quadratic dependence of aphoto-ionization cross-section of F₂ ²⁺(2 Mg)-centers on peak laserlight intensity illustrating high probability of a two-photon absorptionprocess in Al₂O₃:C,Mg crystals;

[0034]FIG. 9 is a wavelength dependence of photo-ionizationcross-section showing an optimum wavelength at about 390 nm to perform“write” operation in Al₂O₃:C,Mg;

[0035]FIG. 10A is a graph showing temperature dependence of an opticalabsorption band at 255 nm (F⁺-centers) and illustrates high thermalstability of trapped charge up to 650° C.;

[0036]FIG. 10B is a graph showing temperature dependence of an opticalabsorption band at 335 nm (F₂ ⁺(2 Mg)-centers) and illustrates highthermal stability of trapped charge up to 650° C.;

[0037]FIG. 10C is a graph showing temperature dependence of an opticalabsorption band at 435 nm (F₂ ²⁺(2 Mg)-centers) and illustrating highthermal stability of trapped charge up to 650° C.;

[0038]FIG. 10D is a graph showing temperature dependence of an opticalabsorption band at 630 nm (F₂ ³⁺(2 Mg)-centers) and illustrating highthermal stability of trapped charge up to 650° C.;

[0039]FIG. 11 is a band diagram illustrating electronic processes in anAl₂O₃ crystal doped with Mg impurity during “write” and “read”operations;

[0040]FIG. 12 is a fluorescent image of a matrix of 3×3 bits written inthe XY plane of the crystal medium with 5 μm increments using CW 405 nmlaser diode and read using a confocal detection scheme and 440 nm CWlaser diode; FIG. 13 is a 500×500 pixel image obtained in fluorescentcontrast after writing 100×100 bits in the XY plane of the crystal with1 μm increments in which bits were written with pulses of blue 411 nmlaser diode and read with 440 nm laser diode;

[0041]FIG. 14 is a bit profile obtained in fluorescent contrast duringX-scan of a crystal showing spatial resolution of the bits written with15 nJ per bit and 1 μm incremental steps

[0042]FIG. 15 is a 400×400 pixel image obtained in fluorescent contrastin the XZ plane of the crystal after writing 3 bits in the X directionwith 5 μm increments. Bits were written with pulses of blue 411 nm laserdiode and read with 440 nm CW laser diode;

[0043]FIG. 16 is a 500×500 pixel image of 3 layers of bits obtained influorescent contrast in the XZ plane of the crystal after writing matrixof 7×3 bits with 5 μm increments in the X direction and 10 μmincremental motion in the Z direction in which bits were written withpulses of blue 411 nm laser diode and read with a 440 nm CW laser diode;

[0044]FIG. 17 is a graph demonstrating multilevel one-bit recording bymeasuring a fluorescent image during XY-scan of Al₂O₃ crystal in which10 bits were written in the volume of the crystal with different numbersof 60 ps “write” laser pulses; and

[0045]FIG. 18 is a graph illustrating dependence of the modulation depthof bits as a function of number of “writing” laser pulses anddemonstrating multilevel one-bit recording.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

[0046] It is advantageous to define several terms before describing theinvention. It should be appreciated that the following definitions areused throughout this application.

Definitions

[0047] Where the definition of terms departs from the commonly usedmeaning of the term, applicant intends to utilize the definitionsprovided below, unless specifically indicated.

[0048] For the purposes of the present invention, the term “writing”refers to the conventional meaning of the term “writing” with respect tostoring information on a data storage medium. In a preferred embodimentof the present invention, information is written to a data storagemedium using a laser beam of one or more frequencies.

[0049] For the purposes of the present invention, the term “writeposition” refers to positioning a data storage medium to a position atwhich the data storage medium may be written to by a laser beam.

[0050] For the purposes of the present invention, the term “modulationdepth of a fluorescent signal” refers to the parameter of the opticaldata storage system determined as the ratio of two fluorescent signalsobtained from the same media spot before and after recording/writing.

[0051] For the purposes of the present invention the term “multilevelrecording” refers to a method of writing information in a storage mediumin which upon readout with a reading beam produces a readout value thatmay be digitized onto several discrete value levels representing morethan one bit of digital information.

[0052] For the purpose of the present invention, the term “write time”refers to the time during which the writing beam is illuminating thespot on the medium to achieve desired change in the fluorescence signalamplitude (modulation depth of fluorescent signal). Such change in thefluorescence signal amplitude may be as low as 1% or as high as morethan 90% depending on the desired modulation depth.

[0053] For the purposes of the present invention, the term “reading”refers to the conventional meaning of the term “reading” with respect toretrieving information stored on a data storage medium. In a preferredembodiment of the present invention, information is read from a datastorage medium using a laser beam of one or more frequencies.

[0054] For the purposes of the present invention, the term “read time”refers to the time a specific storage location is illuminated by theread laser. The read time is equal to the read laser pulse length forstationary media and as a ratio of the reading spot size to the mediumvelocity for moving the medium.

[0055] For the purposes of the present invention, the term “erasing”refers to any of the conventional meanings of the term “erasing” withrespect to digital data storage media. In general, erasing refers torestoring one or more sections of a data storage medium containingstored information to a state those sections had before havinginformation stored in those sections.

[0056] For the purposes of the present invention, the term “physicallyerasing” refers to removing or destroying previously stored informationon a data storage medium.

[0057] For the purposes of the present invention, the term “absorptionband in the region of” or “emission band in the region of” refers to anabsorption or emission band having a peak in the appropriate region.Sometimes the region may be a particular wavelength and sometimes theregion may include a range of wavelengths indicating a possible shift ina band peak position.

[0058] For the purposes of the present invention, the term “crystallinematerial” refers to the conventional meaning of the term “crystallinematerial”, i.e. any material that has orderly or periodic arrangement ofatoms in its structure.

[0059] For the purposes of the present invention, the term “luminescentmaterial” refers to any of the conventional meanings of the term“luminescent material”.

[0060] For the purposes of the present invention, the term “data storagemedium” refers to a medium upon which data may be stored, generally indigital form.

[0061] For the purposes of the present invention, the term “luminescentdata storage medium” refers to a data storage medium that is comprisedin part or in its entirety of a luminescent material.

[0062] For the purposes of the present invention, the term “defect”refers to the conventional meaning of the term “defect” with respect tothe lattice of a crystal, i.e. a vacancy, interstitial, impurity atom orany other imperfection in a lattice of a crystal.

[0063] For the purposes of the present invention, the term “oxygenvacancy defect” refers to a defect caused by an oxygen vacancy in alattice of a crystalline material. An oxygen vacancy defect may be asingle oxygen vacancy defect, a double oxygen defect, a triple oxygenvacancy defect, or a more than triple oxygen vacancy defect. An oxygenvacancy defect may be associated with one or more impurity atoms or maybe associated with an interstitial intrinsic defect such as misplacedinterstitial oxygen atoms. Occupancy of an oxygen vacancy by twoelectrons gives rise to a neutral F-center, whereas occupancy of anyoxygen vacancy by one electron forms an F⁺-center. An F⁺-center has apositive charge, with respect to the lattice. A cluster of oxygenvacancy defects formed by double oxygen vacancies is referred to as anF₂-type center. A cluster of oxygen vacancy defects formed by twoF⁺-centers and charge-compensated by two Mg-impurity atoms is referredto as a F₂ ²⁺(2 Mg)-center.

[0064] For the purposes of the present invention, the term “F-typecenter” refers to any one of the following centers: F-center, F⁺-center,F₂ ⁺-center, F₂ ⁺⁺-center, F₂ ⁺(2 Mg)-center, F₂ ⁺⁺(2 Mg)-center, etc.

[0065] For the purposes of the present invention, the term “colorcenter” refers to the conventional meaning of the term “color center”,i.e. a point defect in a crystal lattice that gives rise to an opticalabsorption of a crystal and upon light excitation produces a photon ofluminescence. A color center, an impurity or an intrinsic defect in acrystalline material creates an unstable species. An electron localizedon this unstable species or defect performs quantum transition to anexcited state by absorbing a photon of light and performs quantumtransition back to a ground state by emitting a photon of luminescence.In a preferred embodiment of the present invention, color centers arepresent in a concentration of about 10¹³ cm³¹ ³ to 10¹⁹ cm⁻³.

[0066] For the purposes of the present invention, the term “luminescencelifetime” or “fluorescence lifetime” refers to a time constant of anexponential decay of luminescence or fluorescence.

[0067] For the purposes of the present invention, the term “wideemission band” refers to an emission band that has full width at halfmaximum bigger than 0.1 eV and is a result of strong electron-phononinteraction. One example of a wide emission band is the wide emissionband around 520 nm.

[0068] For the purposes of the present invention, the term“charge-compensated” refers to a defect in a crystal lattice thatelectro-statically compensates the electrical charge of another defect.For example, in a preferred embodiment of the present invention, Mg andC impurities may be used to charge-compensate one oxygen vacancy defect,two oxygen vacancy defects, a cluster of these defects, etc. comprisingF₂ ²⁺(2 Mg)-centers.

[0069] For the purposes of the present invention, the term “two-photonabsorption or 2PA” refers to a quantum mechanical process of lightabsorption by a color center when two photons have been absorbedsimultaneously or sequentially by the localized electron of the colorcenter and the electron makes a quantum transition into an excited stateor a conduction band of the crystal.

[0070] For the purposes of the present invention, the term “one-photonabsorption or 1PA” refers to a quantum mechanical process of lightabsorption by a color center when only one photon has been absorbed bythe localized electron of the color center and the electron makes aquantum transition into an excited state without having been transferredto a conduction band of the crystal.

[0071] For the purposes of the present invention, the term “laser lightpower density” or “laser light intensity” refers to a physical quantitymeasured as an average light energy of the laser beam propagatingthrough the medium per second and divided by the area of laser beamwaist.

[0072] For the purposes of the present invention, the term“substantially insensitive to room light” refers to a crystallinematerial that does not change significantly its coloration orconcentration of electrons on traps (concentration of unstable species)under ambient light conditions.

[0073] For the purposes of the present invention, the term “capable ofbeing used for long-term data storage” refers to a crystalline materialthat does not change significantly its coloration or concentration ofelectrons on traps (concentration of unstable species) at ambienttemperatures.

[0074] For the purposes of the present invention, the term“photo-ionization cross-section” refers to a parameter having adimension of cm²/J that determines how much light energy per unit areais required to perform photo-ionization of a color center. The largerthe photo-ionization cross-section means less energy per unit area isrequired to perform ionization (recording of the bit).

[0075] For the purposes of the present invention, the term “fluorescenceyield” refers to the parameter determined as a ratio of the number ofphotons emitted by a luminescent material to the number of photonsabsorbed by this luminescent material.

[0076] For the purposes of the present invention, the term “electrontrap” refers to a structural defect in a crystal lattice able to createa localized electronic state and capable of capturing free electronsfrom a conduction band of the crystalline material.

[0077] For the purposes of the present invention, the term “hole trap”refers to a structural defect in a crystal lattice able to create alocalized electronic state and capable of capturing free holes from aconduction band of the crystalline material.

[0078] For the purposes of the present invention, the term “deep trap”refers to an electron or hole trap having a thermal activation energylarger than kT, where T is absolute temperature of the crystal and k isBoltzmann's constant.

[0079] For the purposes of the present invention, the term “efficientdeep trap” refers to a deep trap which is capable of trapping electronsor holes and which has a sufficient capture cross-section.

[0080] For the purposes of the present invention, the term “multileveloptical data storage” or “multivalued optical data storage” refers tothe ability of the data storage system to perform recording and readingof data from the same physical location in the medium with a number ofquantized data or signal levels more than two.

[0081] For the purposes of the present invention, the term “confocaldetection” refers to a method of optically detecting a fluorescencesignal in which the focal plane inside the medium of the opticalread/write head is optically re-imaged onto a plane which contains anaperture or set of apertures having size comparable to or smaller thanthe diffraction limited spot size of the projected spot of fluorescentdata storage medium.

[0082] For the purposes of the present invention, the term “sphericalaberration compensation (SAC)” refers to a technique for compensating orcorrecting the spherical aberration that arises when a high numericalaperture objective (of NA of at least 0.35) is focused to a differentdepth inside the volume of the storage medium. The spherical aberrationcorrection allows to maintain diffraction limited spot size over largerdepth of the medium, preferably larger than 500 microns, via dynamicallychanging the focusing lens properties depending on the depth of thefocus inside the medium achieving diffraction limited spot size at thefocusing depth.

Description

[0083] The need for high capacity and high transfer rate computerdevices for massive data storage has stimulated a search for new typesof media that are able to exist in two or more stable configurations. Bytransferring the storage medium from one configuration into another, onemay write and erase the bit of information, whereas by analyzing theconfiguration of the medium, the reading of the bit is realized. A largenumber of materials and techniques have been suggested for data storageand data processing, but only a few of these techniques have found apractical application. Because of the large number of requirements, itis extremely difficult to develop a medium for optical data storagedevices, which preferably meets all these requirements. The followingarticles, the contents and disclosures of which are hereby incorporatedby reference, describe several of the techniques that have beenattempted: International Symposium on Optical Memory and Optical DataStorage 2002, Technical Digest Publication of IEEE, Catalog #02EX552(July 2002); Optical Data Storage 2001, Proceedings of SPIE, Vol. 4342(2001); Optical Data Storage 2000, Proceedings of SPIE, Vol. 4090(2000); International Symposium on Optical Memory and Optical DataStorage 1999, SPIE, Vol. 3864 (1999); Advanced Optical Data Storage:Materials, Systems, and Interfaces to Computers, Proceedings of SPIE,Vol. 3802 (1999); and K. Schwartz, The physics of optical recording,Springer-Verlag, Germany (1993), the entire contents and disclosures ofwhich are hereby incorporated by reference.

[0084] Some of the most important requirements that data storage devicespreferably meet are: an ability to repeatedly write, read, and erase thedata (>10⁶ cycles); a high density of bits per unit volume or area(>10¹¹ cm⁻³); a high data transfer rate (>10⁷ bit/s); a minimum accesstime (<10⁻² s); a long lifetime for the storage medium (>10 years) andnon-volatility of the information stored in the storage medium; anenvironmental stability of medium characteristics; safety of the storeddata; and an ability to accomplish data error correction.

[0085] Several methods have been attempted to provide storage devicesthat might be able to compete with or replace conventional magneticinduction methods to achieve desirable characteristics. Among themethods attempted have been to use: magneto-optic and electro-opticeffects (Pockels effect, Kerr effect, Faraday effect, photorefractiveeffect, etc.), the photochromic effect in dye polymers and inorganiccrystals, and phase transformation in the storage medium at the spotbeing heated with a laser beam. Some of these methods have beensuccessfully realized in the form of phase change media in the form ofCD-RW and DVD-RW, and magneto-optical WREM discs and drives that arealready on the market. Other methods, such as near-field, solidimmersion lens recording, and atomic force microscopy are merelycontemplated, see Alternative Storage Technologies Symposium 2001,Monterey Calif., Jun. 26, 2001, the entire contents and disclosure ofwhich is hereby incorporated by reference.

[0086] Most of the conventional techniques mentioned above that use 2D(thin film) media are approaching a fundamental limit of storage densitycaused by a minimum achievable focused laser light spot or in the caseof magnetic recording by thermal instabilities of magnetic domain walls(super paramagnetic effect). The most promising way to overcome theselimitations may be to use volumetric (3D-space) recording. Among 3Dtypes of data storage, the types of data storage that have beeninvestigated, most have been in the area of digital holography, seeHolographic data storage, (eds.: H. J. Coufal, D. Psaltis, G.Sincerbox), Springer 2000, p. 488, and volumetric multilayer single-bitrecording, see Confocal and Two Photon Microscopy, Foundations,Applications, and Advances, (ed. Alberto Diaspro) Wiley-Liss, 2002, p.567; the entire contents and disclosures of the above references arehereby incorporated by reference.

[0087] Several kinds of materials, such as photopolymers, photochromicmaterials and photorefractive crystals, have been proposed as possiblerecording media with a confocal detection scheme when one bit in thevolume of the medium may be written as a local refractive index changeusing two-photon absorption (2PA) of a high peak-power short pulse laserbeam and the recorded data is read by measuring the change in reflectionof the read laser light, see U.S. Pat. No. 5,289,407 to Strickler, etal.; James H. Strickler, Watt W. Webb, Three-dimensional optical datastorage in refractive media by two-photon point excitation, OpticsLetters, Volume 16, Issue 22, 1780, November 1991; Y. Kawata, H.Ishitobi, S. Kawata, Use of two-photon absorption in a photorefractivecrystal for three-dimensional optical memory, Optics Letters, Volume 23,Issue 10, 756-758, May 1998; A. Toriumi, J. M. Herrmann, S. Kawata,Nondestructive readout of a three-dimensional photochromic opticalmemory with a near-infrared differential phase-contrast microscope,Optics Letters, Volume 22, Issue 8, 555-557, April 1997; M. Ishikawa, Y.Kawata, C. Egami, O. Sugihara, N. Okamoto, M. Tsuchimori, O. Watanabe,Reflection-type confocal readout for multilayered optical memory, OpticsLetters, Volume 23, Issue 22, 1781-1783, November 1998; A. Toriumi, S.Kawata, M. Gu, Reflection confocal microscope readout system forthree-dimensional photochromic optical data storage, Optics Letters,Volume 23, Issue 24, 1924-1926, December 1998; Min Gu, Daniel Day, Useof continuous-wave illumination for two-photon three-dimensional opticalbit data storage in a photo-bleaching polymer, Optics Letters, Volume24, Issue 5, 288-290, March 1999; Yoshimasa Kawata, Takuo Tanaka,Satoshi Kawata. Randomly accessible, multilayered optical memory with aBi₁₂SiO₂₀ crystal, Applied Optics-IP, Volume 35, Issue 26, 5308-5311,September 1996; Daniel Day, Min Gu, Andrew Smallridge, Use of two-photonexcitation for erasable rewritable three-dimensional bit optical datastorage in a photo-refractive polymer, Optics Letters, Volume 24, Issue14, 948-950, July 1999; Y. Shen, J. Swiatkiewicz, D. I. Jakubczyk, F.Xu, P. N. Prasad, R. A. Vaia, B. A Reiihardt, High-Density Optical DataStorage With One-Photon and Two-Photon Near-Field FluorescenceMicroscopy, Applied Optics, Volume 40, No. 6, 938-940, February 2001; T.Wilson, Y. Kawata, S. Kawata, Readout of Three-Dimensional OpticalMemories, Optics Letters, Volume 21, No. 13, 1003-1005, July 1996; H.Ueki, Y. Kawata, S. Kawata, Three-Dimensional Optical Bit-MemoryRecording and Reading With a Photorefractive Crystal: Analysis andExperiment, Applied Optics, Volume 35, No. 14, 2457-2465, May 1996; MinGu, Confocal Readout of Three-Dimensional Data Bits Recorded by thePhotorefractive Effect Under Single-Photon and Two-Photon Excitation,Proceedings of the IEEE, Volume 87, No. 12, 2021-2029, December 1999,the entire contents and disclosures of which are hereby incorporated byreference.

[0088] One-bit micro-holograms were suggested as the way to increase bitreflectivity and signal-to-noise and carrier-to-noise ratio (SNR andCNR, respectively), see U.S. Pat. No. 6,322,931 to Cumpston, et al.;U.S. Pat. No. 6,322,933 to Daiber, et al.; H. J. Eichler, P. Kuemmel, S.Orlic, A Wappelt, High-Density Disk Storage by MultiplexedMicroholograms, IEEE Journal of Selected Topics in Quantum Electronics,Volume 4, No. 5, 840-848, September/October 1998; Y. Kawata, M. Nakano,Suk-Chun Lee, Three-Dimensional Optical Data Storage UsingThree-Dimensional Optics, Optical Engineering, Volume 40, No. 10,2247-2254, October 2001, the entire contents and disclosures of whichare hereby incorporated by reference.

[0089] Several types of photochromic materials have been proposed for 3Done-bit optical data storage: organic fluorescent dyes dispersed in apolymer matrix, which undergo chemical or structural conformation,diffusion and polymerization as a result of illumination. Two-photonabsorption in fluorescent photopolymers and confocal detection schemeshave also been used, see U.S. Pat. No. 5,325,324 to Rentzepis, et al.;D. A. Parthenopoulos and P. M. Rentzepis, Three-Dimensional OpticalStorage Memory, Science, Vol. 245, pp. 843-845, 1989; Daniel Day, MinGu, Effects of Refractive-Index Mismatch on Three-Dimensional OpticalData-Storage Density in a Two-Photon Bleaching Polymer, AppliedOptics-IP, Volume 37, Issue 26, 6299-6304, September 1998; Mark M. Wang,Sadik C. Esener, Three-Dimensional Optical Data Storage in a FluorescentDye Doped Photopolymer, Applied Optics, Volume 39, No. 11, 1826-1834,April 2000; E. P. Walker, X. Zheng, F. B. McCormick, H. Zhang, N.-H.Kim, J. Costa, A. S. Dvornikov, Servo Error Signal Generation for2-Photon Recorded Monolithic Multilayer Optical Data Storage, OpticalData Storage 2000, Proceedings of SPIE Vol. 4090, pp. 179-184, 2000; H.Zhang, A. S. Dvornikov, E. P. Walker, N.-H. Kim, F. B. McCormick, SingleBeam Two-Photon-Recorded Monolithic Multi-Layer Optical Disks, OpticalData Storage 2000, Proceedings of SPIE, Vol. 4090, pp. 174-178, 2000; Y.Zhang, T. D. Milster, J. Butz, W. Bletcher, K. J. Erwin, E. Walker,Signal, Cross Talk and Signal to Noise Ratio in Bit-Wise VolumetricOptical Data Storage, Technical Digest of Joint International Symposiumon Optical Memory and Optical Data Storage, IEEE Catalog, No. 02EX552,pp.246-248, 2002; E. P. Walker, W. Feng, Y. Zhang, H. Zhang, F. B.McCormick, S. Esener, 3-D Parallel Readout in a 3-D Multilayer OpticalData Storage System, Technical Digest of Joint International Symposiumon Optical Memory and Optical Data Storage, IEEE Catalog, No. 02EX552,pp. 147-149, 2002; and Ingolf Sander (Constalation 3D, Inc.) Fluorescentmultilayer technology, In: Alternative Storage Technologies Symposium2001, Monterey Calif., Jun. 26, 2001, the entire contents anddisclosures of which are hereby incorporated by reference.

[0090] Using luminescent materials as data storage media is especiallyattractive because of their ability to realize multilevel (ormultivalued) optical data storage. Luminescent response is proportionalto the product of the energy deposited in the medium during “writing”and “reading”. If the concentration of defects undergoing electronictransition in the volume corresponding to one bit of information islarge enough, then that element of the light-sensitive medium may beused in a “gray scale” mode and the optical data storage system may beused as a multilevel (or multivalued) data storage system. The potentialstorage capacity is increased proportionally to the number of datalevels reliably achieved. The total linearity of luminescent responsemay stretch over several orders of magnitude. Different logical statesof the medium may be represented by different intensities of theluminescent signal and digitized using thresholding electronic circuits.In practice, 10 levels of fluorescent intensity may be achieved bychanging the energy or the time duration of the laser “writing” beam. Anincreased density of data storage is one of the main potentialadvantages of the luminescent techniques of the present invention.

[0091] Similar approaches to writing to and reading from a data storagemedium have been demonstrated in silver-doped photoluminescent glassesused in radiation dosimetry, see B. Lommier, E. Pitt, A. Scharmann,Optical creation of radiophotoluminescence centers in dosimeter glass bytwo-photon absorption, Radiat. Prot. Dosim. Vol. 65, No. 1-4, pp.101-104(1996), the entire contents and disclosure of which is herebyincorporated by reference. Two-photon UV excitation (“writing”) producesa photoluminescence signal that may be repeatedly “read” with the samelaser, but at lower power, without measurable erasure of information.However, how such data may be “erased” without heating the medium is notclear. Complications may also be caused by the long-term process ofdiffusion and luminescent center transformation that lead to a“build-up” of luminescent signal.

[0092] A promising way to overcome the limitations of conventional datastorage systems is to use volumetric or 3D recording. Among3D-technologies, multilayer one-bit recording with two-photon absorption(2PA) has some clear advantages. The probability of two-photonabsorption is proportional to a square of laser light intensity.Two-photon absorption allows one to perform photo-ionization orphoto-transformation of a photosensitive medium only in the vicinity ofa tightly focused laser beam without affecting the surrounding volume ofthe material. The size of one three-dimensional bit or voxel writtenusing 2PA may be made as small as 1×1×3 μm. Extremely high storagedensity of up to 10 Tbits/in³ is expected.

[0093] An apparatus built according to the present invention isillustrated by FIG. 1, described in more detail below. Two lasers basedon blue laser diodes are used to write and to read the data from astorage medium. Two-photon absorption from a more powerful CW laserhaving a shorter wavelength is used for recording the data. High NA ofthe objective lens allows for the high intensity of the laser lightneeded for 2PA to be achieved and for the formation of a diffractedlimited recorded bit size. One-photon-induced fluorescence induced bylow energy and longer wavelength laser and a confocal detection schememay be used for reading the data. Confocal detection allows one tosignificantly reduce cross-talk between adjacent bits, tracks, andlayers with the purpose of achieving a desirable signal-to-noise ratio(SNR).

[0094] An important objective of the present invention is to provide amethod and apparatus that performs writing operation using highprobability of 2PA and makes the reading operation non-destructive bydecreasing the probability of 2PA during the readout. At the same timethe method and apparatus according to the present invention use 1PA andlaser-induced fluorescence during readout at a highest possible levelneeded to achieve acceptable SNR and high data transfer rate.

[0095] Most of the problems with the various storage media describedabove have been related to inadequate material properties. Most of thephotopolymers suggested for one-bit or holographic data storage showhigh sensitivity but exhibit strong dimensional shrinkage. Most of thephoto-sensitive polymers may be used only as WORM media (write once,read many times), whereas rewritable photopolymers are still unstableand show significant fatigue when write-read cycles are repeated manytimes. Even write-once fluorescent photopolymers show strong reductionof fluorescent output signals when read repeatedly. An additionalobstacle for most suggested photopolymers and photorefractive crystalstested for volumetric one-bit recording is a necessity of using afemto-second high peak power Ti-sapphire laser to achieve efficienttwo-photon absorption, because a Ti-sapphire laser is big, expensive andat the time are suitable mostly for laboratory demonstration.

[0096] Therefore, utilization of an efficient and stable inorganicphotochromic fluorescent material for one-bit optical recording andreading is an objective of the present invention.

[0097] The low thermal energy depth of the traps responsible forcapturing electrons produced by ionization with the laser light duringwriting causes thermally stimulated release of electrons from traps atambient temperatures and fading of the stored data. This is generallynot acceptable, especially for multivalued storage, which requiresprecise digitization of the analog luminescent signal. Furthermore, thechemical instability of some luminescent materials and their sensitivityto oxidization and humidity when contacted with air require the use ofprotective layers. In some organic fluorescent materials, dimensionalstability is a significant problem because of material shrinkage as aresult of photochemical transformation and polymerization.

[0098] Electronic transitions in solids caused by light excitation withrelaxation times on the order of 10⁻¹² to 10⁻⁹ s are fundamentally veryfast and are considered among the most promising quantum systems formassive optical data storage. Luminescence decay time after a pulse oflaser stimulation determines the time needed for retrieval of each bitof information and the maximum achievable data transfer rate.

[0099] To achieve more stable and reliable data storage and opticalprocessing, one should use chemically, mechanically and thermally stableluminescent materials with deep traps and luminescent centers. Toproduce such deep centers, one needs to use a wide gap dielectric.Furthermore, these “thermally” and “optically” deep traps require ashorter wavelength of laser light for excitation (“writing”),stimulation (“reading”) and restoration (“erasing”). For opticalrecording, the minimum light spot diameter is equal to: d≈0.5λ/NA, whereNA is the numerical aperture of the optical head. Therefore, blue and UVlasers have a clear advantage against IR lasers for achieving higherstorage densities. The latest developments in blue and UV solid statelasers, based on heterostructures of wide gap semiconductors like GaA1Ncreate a real possibility for use of materials with wide energy gaps.

[0100] The low energy depth of the traps responsible for theaccumulation of the charge carriers leads to the thermally stimulatedrelease of electrons at ambient temperatures and to fading of the storeddata. This is not acceptable, especially for multilevel data storage,which requires precise digitizing of the analog luminescent signal.Furthermore, the chemical instability and sensitivity to oxidization andhumidity when contacted with air require the use of protection layers.

[0101] Corundum or sapphire (α-Al₂O₃) is an important technologicalmaterial in many optical and electronic applications. It is used as ahost material for solid-state lasers, as optical windows, as a substratematerial in semiconductor epitaxial growth and, more recently, as aradiation detector see M. S. Akselrod, V. S. Kortov, D. J. Kravetsky, V.I. Gotlib, Highly Sensitive Thermoluminescent Anion-Defective α-Al2O3:CSingle Crystal Detectors, Radiat. Prot. Dosim., Vol. 32(1), pp.15-20(1990). In spite of excellent dosimetric properties of carbon-dopedAl₂O₃ with oxygen vacancies, the luminescent centers in this material(F-centers) have a very long luminescence lifetime (35 ms). However,α-Al₂O₃ is unacceptable for fluorescent one-bit recording applicationsrequiring a high data transfer rate. Known Al₂O₃ materials also do nothave absorption bands that may undergo photochromic transitions suitablefor volumetric data storage applications.

[0102] With respect to the new Al₂O₃ crystalline material described inU.S. application Ser. No. 10/309,021, filed Dec. 4, 2002, and U.S.application Ser. No. 10/309,179, filed Dec. 4, 2002, the entire contentsand disclosures of which are hereby incorporated by reference, importantfeatures of this material utilized in the present invention are theelectronic and optical properties of a storage phosphor and its defectstructure. The Al₂O₃:C,Mg crystalline material has color centersabsorbing light, stable traps of electrons and holes and its luminescentcenters have a short luminescence lifetime.

[0103] A simplified scheme illustrating how the crystalline fluorescentmaterial may be used in an optical data recording and retrieving driveis illustrated by FIG. 1. In scheme 100, a 405 nm “write” laser beam 102and 440 nm “read” laser beam 104 are produced by two diode lasers 106and 108, respectively. Laser beams 102 and 104 are directed on a singlecrystalline disk 110 made of Al₂O₃:C,Mg through a flipping mirror 112,dichroic mirror 114, and a high NA focusing objective lens 116. Disk 110spins as shown by arrow 118 or moves by a 3D translation stagerepresented by arrows 119. An optical pick-up head that combinesobjective lens 116, dichroic mirror 114, focusing lens 120, a confocalpinhole 122, and a photodetector 124 slides along the radius of the disk110. Selection of the focal depth of the bit 126, a certain data layerwithin 3D volume of the disk and correction of spherical aberrations areperformed by moving an additional optical component of an opticalpick-up head (not shown). A photodetector 124 is used to monitorlaser-induced fluorescence 128 during writing and reading. A greenfluorescence 126 is collected by objective lens 116, reflected by thedichroic mirror 114, focused by lens 120 on confocal pinhole 122, anddetected by a photon counter or a digital oscilloscope 130 interfacedwith a computer 132.

[0104] In the scheme shown in FIG. 1, two-photon absorption during a“write” operation allows for very tight localization of thephoto-ionization process in the focal spot of the laser beam and 3Dconfinement of the written bit (green fluorescence 126). One-photonexcitation of the fluorescent light from the medium using “read” laserlight of a longer wavelength and a confocal fluorescence detectionscheme allows one to perform non-destructive reading of bits multipletimes.

[0105] The method described in the present invention is made possibledue to the unique optical properties of Al₂O₃:C,Mg. The primaryinformation storage process in Al₂O₃ is photoionization, followed by thesubsequent capture of the excited electronic charge by trapping centers.Thus, for the efficient storage of information, it is necessary thatAl₂O₃ crystals contain both light absorbing color centers and defectscapable of trapping electrons. High quantum yield of fluorescence isalso required for one-bit confocal recording. According to a preferredembodiment of the present invention volumetric recording of bits isperformed using 2PA, whereas non-destructive reading operation utilizes1PA and fast laser induced fluorescence.

[0106] The Al₂O₃ crystalline materials produced according to a methoddescribed in U.S. application Ser. No. 10/309,021, filed Dec. 4, 2002,and U.S. application Ser. No. 10/309,179, filed Dec. 4, 2002, the entirecontents and disclosures of which are hereby incorporated by reference,and utilized in the present invention include several types of oxygenvacancy defects and are doped with carbon and magnesium impurities andcan be grown by those skilled in the art using any conventional crystalgrowth method (for example using the Czochralski method). Thecrystalline material utilized in the present invention is characterizedby several optical absorption (OA) bands: at 205, 230, 255, 335, 435,and 630 nm (FIG. 2). The blue absorption band at 435 nm is responsiblefor the visible green coloration of the crystal. One important featureof the new aluminum oxide material is a high concentration of single anddouble oxygen vacancies in the form of neutral F-centers as well as F⁺and F₂ ²⁺ centers, charge-compensated by the nearby Mg-impurity atoms.

[0107] An F⁺-center is an oxygen vacancy with one electroncharge-compensated by one Mg²⁺-ion and is denoted as an F⁺(Mg)-center.This center is characterized by at least two absorption bands at 230 and255 nm (FIG. 2) and has a luminescence band at 330 nm with a lifetime ofless than 5 ns. A cluster of two of these defects forms an aggregatevacancy defect composed of two F⁺-centers and two Mg-impurity atoms.This aggregate defect with two localized electrons, denoted here as F₂²⁺(2 Mg), is favorable for optical data storage. It is responsible for ablue absorption-excitation band at 435 nm (FIG. 3), produces a greenfluorescence band at 520 nm, and has a short lifetime equal to 9±3 ns(FIG. 4).

[0108] Exposure of an Al₂O₃:C,Mg crystal having oxygen vacancy defectsto high intensity laser light of appropriate wavelength results inconversion of the same structural defect from one charged state intoanother. For example, F₂ ²⁺(2 Mg)-centers are converted into F₂ ⁺(2Mg)-centers with 430 nm illumination (FIG. 5) and may be converted backwith 335 nm pulsed laser light. After photochromic transition induced byblue laser light, Al₂O₃:C,Mg crystals exhibit an absorption/excitationband at 335 nm (FIGS. 5 and 6), a broad fluorescent emission at 750 nm(FIG. 6) with relatively fast decay time equal to 80±10 ns (FIG. 7).

[0109] In a preferred embodiment of the present invention a two-photonabsorption (2PA) process is utilized for recording the information inthe volume of Al₂O₃:C,Mg. Usually 2PA is considered as a process ofsimultaneous absorption by the luminescence center of two photons. Thesum of energies of these two photons is preferably enough to performexcitation of the luminescent center whereas energy of only one photonis not sufficient for the excitation transition. 2PA in that case isperformed through the virtual (non-existing) quantum energy state ofdefect and the probability of it is very low. To perform 2PA, femto- orpicosecond laser pulses with a power density on the order of 100 MW/cm²are required. A very important trait of Al₂O₃:C,Mg, which enablesso-called “sequential” 2PA, is that defects absorbing the laser lightand producing the fluorescence have an excited state located deep in theenergy gap. The lifetime of this excited state is sufficiently long tosignificantly increase the probability of a second photon absorptionneeded for photo-ionization and data recording. At the same time thelifetime of the excited state for this color center is short enough toallow fast reading of a fluorescent signal with a high data transferrate.

[0110] The evidence of preferred two-photon absorption in aggregateoxygen vacancy defects is provided by quadratic dependence of thephoto-ionization cross-section of these centers versus laser lightintensity (see FIG. 8). Photo-ionization cross-section for the 2PAprocess is inversely proportional to the decay constant and directlyproportional to the product of the absorption cross-sections of theground and excited states. Wavelength dependence of photo-ionizationcross-section of F₂ ²⁺(2 Mg) centers is shifted to a shorter wavelengthin comparison with the 1PA band (FIGS. 3 and 9) and is anotherindication of the 2PA process in Al₂O₃:C,Mg crystals.

[0111] Erasing of written data and restoration of original opticalabsorption (coloration) and fluorescence of the storage medium accordingto the present invention can be achieved optically or thermally.Al₂O₃:C,Mg crystalline material includes deep traps of charge. Thesedeep traps of charge have a delocalization temperature about 600° C. to700° C. and have a concentration about 10¹³ to 10¹⁷ cm⁻³. Thedelocalization temperature of these deep traps was found from theoptical absorption experiment (see FIGS. 10A, 10B, 10C and 10D) withstep annealing of the Al₂O₃:C,Mg crystal after it was illuminated with a430 nm pulsed laser light that is equivalent to a “write” operation inthe optical data storage system. Optical absorption bands of F⁺-centersat 255 nm and F₂ ²⁺(2 Mg)-centers at 435 nm increase their intensitiesand restore their original intensity in the temperature region between600 and 700° C. The opposite trend may be seen in the same temperaturerange for 335 nm band of F₂ ⁺(2 Mg)-centers and 630 nm band of F₂ ³⁺(2Mg)-centers indicating that these centers convert into F₂ ²⁺(2Mg)-centers during annealing. 630 nm absorption band of F₂ ³⁺(2Mg)-centers appears only after “write” operation with the pulsed bluelaser light and is not visible in the absorption spectrum of the freshAl₂O₃:C,Mg crystal shown in FIG. 2.

[0112] Optical erasure of recorded bits by reverse photochromictransformation and restoration of the 435 nm absorption band and 520 nmfluorescence can be achieved according to another preferred embodimentof the present invention using sequential illumination of the Al₂O₃:C,Mgcrystal with 205±30 and 335±30 nm laser light corresponding toabsorption bands of F and F₂ ⁺(2 Mg) centers. Illumination with thelight at 205±30 nm corresponding to an F-absorption band performsphoto-ionization of F-centers and generates free electrons. These freeelectrons can be captured by the defects ionized during recording. Inparticular these electrons are captured by deep hole traps and F₂ ²⁺(2Mg) and F₂ ³⁺(2 Mg)-centers. The goal of one of the preferredembodiments of the present invention is to convert as many double oxygenvacancy defects as possible into a F₂ ⁺(2 Mg) charge state. Thesedefects are characterized by an intensive absorption band in the regionof 335 nm. Ionization of F centers can be performed by either coherentlaser light or incoherent light, produced for example by deuterium ofxenon lamps. Subsequent illumination of the crystal with the high powerdensity laser light having 335±30 nm performs 2PA on F₂ ⁺(2 Mg) centers,converts them into F₂ ²⁺(2 Mg)-centers. The described optical procedurerestores a 435 nm absorption band and 520 nm fluorescence characteristicto these defects and restores original green coloration of the Al₂O₃:C,Mg crystal.

[0113] Preferred electronic processes during “write” and “read”operation in the utilized Al₂O₃:C,Mg material of the present inventionare explained using a band diagram as in FIG. 11. A preferred dopedAl₂O₃ material of the present invention for use as a data storage mediummay be formed to contain a high concentration of trapping sites andfluorescent centers with precisely desirable characteristics. Datastorage media generally exist in at least two stable physical statesassigned correspondingly to “0” and “1” states. An initial configuration(logical 0 state) of as-received or erased Al₂O₃ medium has a highconcentration of F₂ ²⁺(2 Mg)-centers, characterized by an intensiveabsorption band in the region of 435±5 nm.

[0114] By illumination with the writing laser light (“write” beam”) ofthe appropriate photon energy hv₁ (or wavelength λ₁) and intensity,which is high enough to ionize the above described crystal defects, onemay produce free electrons to be trapped in pre-existing electronicdefects. The traps in Al₂O₃:C,Mg are deep enough to keep the chargecarriers for a long time at ambient temperature without being thermallyreleased. This second state of a quantum system is now in a metastable“charged” configuration (logical “1” state). To “read” the state of themedium, the stimulation light of the same as writing light or anotherphoton energy hv₂ (or wavelength λ₂) is applied and fluorescent photonof energy hv₃ (or wavelength λ₃) is detected. In case of fluorescentone-bit recording, a written bit produces reduced fluorescence intensitywhereas an unwritten spot produces original intensive fluorescence.

[0115] Electronic defects in wide gap dielectrics like Al₂O₃ arecharacterized by the energy levels of their ground and excited states.If the excited state of the electronic defect is located close or withinthe conduction band, the defect may be ionized by one-photon absorption.A different situation takes place when the excited state is located deepwithin the energy gap of the crystal. Absorption of one photoncorresponding to the energy transition between ground and excited statesof the electronic defect results in a localized transition followed bynon-radiative and radiative decay (fluorescence). This one-photonabsorption process is nondestructive and may be used for readinginformation.

[0116] To remove an electron from such a deep defect and to change theluminescent properties of the particular defect in the crystal,simultaneous absorption of two photons is used in the present invention.The first absorbed photon excites the electron of the above describedelectronic defect into its excited state, whereas the second photontransfers the electron from the above described excited state within theenergy gap into the conduction band. An electron in the conduction bandis now delocalized (removed from its original defect site) and may betrapped by another defect.

[0117] Writing the data may be performed (see FIG. 11) using two-photonabsorption of 435±40 nm blue laser light by the F₂ ²⁺(2 Mg)-centersdescribed above. The first photon excites one of the two electronslocalized on the center to its excited state while the second photonperforms the second transition between the excited state and theconduction band, thus performing photo-ionization of the center. Thefinal stage of the writing process is localization (or trapping) of theabove described photoelectron by another defect, namely by another F₂²⁺(2 Mg)-center, or by F⁺(Mg)-center or by carbon impurity. The resultof these photochromic transformations is (a) creation of another chargedstate of the aggregate defect, F₂ ⁺(2 Mg)-center, having three localizedelectrons and characterized by a UV absorption band at 335 nm, or (b)creation of a neutral F-center with a UV absorption band at 205 nm, or acarbon related trap responsible for 190° C. TL peak. All three processesresult in formation of optically deep and thermally stable electronicstates and may be used in a preferred embodiment of the presentinvention for long-term data storage. The first process (a) has a higherprobability that was determined from the efficiency of photo-conversionof optical absorption bands. As a result of photo-ionization, an F₂²⁺(Mg)-center converts into F₂ ³⁺(Mg) that has an absorption band at 620nm and the released electron is trapped by another F-₂ ²⁺(Mg)-centerconverting it into an F₂ ⁺(Mg)-center having three localized electronsand characterized by an absorption band at 335 nm and an emission bandat 750 nm.

[0118] The present invention provides two types of fluorescent processesfor reading data (see FIG. 11). The Type 1 or “negative” processinvolves stimulation of original green fluorescence of F₂ ²⁺(2Mg)-centers using blue laser light excitation at 435±40 nm. Theintensity of this excitation is preferably significantly reduced toavoid two-photon absorption, but sufficient to generate greenfluorescence enough for the reliable detection of information. Smallvolumes of Al₂O₃ crystal (voxels) subjected to two-photon ionizationduring writing show reduced or no fluorescence, whereas the unwrittenvoxels show high intensity of green fluorescence.

[0119] A type 2 readout process of the present invention, the so-called,“positive” readout process, involves using laser excitation at 335±30 nmto stimulate the fluorescence of F₂ ⁺(2 Mg)-centers created duringrecording. The intensity of this excitation also is preferablysignificantly reduced to avoid two-photon absorption. The intensity offluorescence of F₂ ⁺(2 Mg)-centers in the region of 750 nm having an 80ns lifetime is used as a measure of data during a “readout” process forbinary or multilevel data storage.

[0120] Writing the data on the aluminum oxide medium of the presentinvention using 2PA and reading of data from the same medium may beperformed by means of laser induced fluorescence (LIF) and is preferablyachieved through electron motion and electronic transitions. No phasetransformation or other structural changes happen during “write” or“read” operations. This makes the data recording and retrievingprocesses extremely fast and reproducible.

[0121] A process of writing data according to a preferred embodiment ofthe present invention will now be described. First the data storagemedium in the form of a Mg-doped anion-deficient Al₂O₃ single crystal ismoved to a desired position with respect to the diffraction limitedlaser “write” beam, focused on a predetermined depth of the medium bymeans of mechanical motion of the medium and/or the adjustablecomponents of the optical head. Spherical aberration compensation of thefocused laser beam is also performed at this stage by means ofmechanical motion of the optical component or by electro-opticalcomponent based for example on liquid crystal phase shifter.

[0122] Then, the data storage medium is illuminated with theabove-described focused beam of writing laser light having wavelength λ₁for the period of time equal to a write time t₁. The above described“write” wavelength λ₁ is in the range of 370-490 nm with a morepreferred wavelength equal to 390 nm. The sequential two-photonabsorption process described in the present invention can be achieved atlaser intensity of higher than 1 kW/cm². The “write” time is in therange of 0.1 ps to 1 ms with a more preferred time equal to 10 ns. Theresult of the writing operation is ionization and photo-conversion of F₂²⁺(2 Mg)-centers into F₂ ³⁺(2 Mg)-centers:

F ₂ ²⁺(2 Mg)+2hv ₁ =F ₂ ³⁺(2 Mg)+e ⁻

[0123] Electrons released from F₂ ²⁺(2 Mg)-centers as a result of thephoto-ionization process are captured by deep traps and other nearby F₂²⁺(2 Mg)- and F⁺-centers:

F ₂ ²⁺(2 Mg)+e ⁻ =F ₂ ⁺(2 Mg)

F ⁺(Mg)+e ⁻ =F(Mg)

[0124] The above-described deep trapping sites are able to storeinformation almost indefinitely.

[0125] The present invention provides three preferred modes of readingdata using a confocal laser induced fluorescent detection scheme. Thefirst mode reads the data with the same laser beam of wavelength λ₁ usedfor “writing”, but with significantly reduced intensity and time ofillumination to avoid two-photon absorption and erasure of stored data.In the second mode of operation, the reading laser beam has a wavelengthλ₂ longer than λ₁, but it is still within the absorption band of F₂ ²⁺(2Mg)-centers. For example, wavelength λ₂ is selected to be 460 nm. Alonger wavelength further reduces the probability of 2PA and allows forhigher laser light intensity for excitation of fluorescence and providesbetter signal-to-noise ratio (SNR). These two modes of “read” operationutilize a fluorescent emission band of F₂ ²⁺(2 Mg)-centers in the regionof 520 nm (FIG. 3). Lifetime of this fluorescence is 9±2 ns (FIG. 4) andis fast enough to achieve a 100 Mbit/s data transfer rate. A strongfluorescence signal corresponding to a 0 binary state indicates that no“write” operation was performed on the particular bit.

[0126] These two first modes of reading may be referred to as “negative”types of operation. The third mode of reading operation utilizesfluorescence of F₂ ⁺(2 Mg)-centers (three electrons occupying theaggregate defect) created as a result of trapping the electron by the F₂²⁺(2 Mg)-centers during writing operation:

F ₂ ²⁺(2 Mg)+e ⁻ =F ₂ ⁺(2 Mg)

[0127] F₂ ⁺(2 Mg)-centers may be excited in their absorption band in theregion of 335 nm or in F⁺-center absorption band at 255 nm. Emission ofthese centers is in the infrared region and is in the region of 750 nm(see FIG. 6 for details of excitation and emission spectra). Thelifetime of the 750 nm emission is 80±10 ns (see FIG. 7) and is shortenough for a data transfer rate up to 10 Mb/s.

[0128] A preferred reading operation of the present invention will nowbe described. First, the above described data storage medium is moved toa desired position with respect to a focused “read” laser beam. Theabove-described read laser beam has a wavelength 2 is in the range of390-500 nm with a more preferred wavelength equal to 460 nm. Then, theabove described data storage medium is illuminated with the abovedescribed focused beam of “read” light for the period of time equal to a“read” time t₂. The above described “read” time t₂ is in the range of0.1 ns to 1 ms with the more preferred time in the range between 2 and15 ns most preferred time equal to 5 ns. The laser-induced fluorescenceproduced by the Al₂O₃ data storage medium is then measured using aphotodetector. The above-described LIF is the “data” light at the thirdwavelength 23 in the region of 750 nm and is in the range from 620 nm to880 nm. The above-described fluorescent signal is then processed toobtain the value of the stored data.

[0129] In another preferred embodiment of the present inventionfluorescence intensity from the recorded bit is inversely proportionalto the amount of energy (or number of “write” laser pulses) delivered tothe bit during the recording stage (FIGS. 17 and 18). It can bedigitized for binary or multilevel types of data and thus can be usedfor further increase of density of data storage.

[0130] The present invention also allows parallel processing of multiplemarks on the storage medium for further increase of “write” or “read”rate and data storage density. Parallel processing is one of the mainadvantages of optical data recording. One may use one-dimensional ortwo-dimensional arrays of lasers and photo-detectors (CCD chips or CMOSdetectors).

[0131] The storage medium of the present invention also providesthermal, temporal and environmental stability of the medium and storeddata. The common problem for fluorescent and photorefractive datastorage medium is the thermal instability and result in thermal erasureof stored information. Al₂O₃ doped with carbon and magnesium exhibitsextremely good thermal and temporal stability of information stored aselectrons trapped on localized states formed by oxygen vacancy defectsin the crystal structure. Lifetime of the charge carriers on trapsdepends on storage temperature. The higher the temperature, the smallerthe lifetime. The deeper the traps—the longer the storage time. Most ofthe trapped electrons are associated with a 650° C. trap that hasextremely high thermal and optical depth. Al₂O₃ crystals are verymechanically, chemically and optically stable and do not showdegradation of performance for years. It was also shown that therecorded data is not erased by conventional room light illumination andthe medium does not require light protection.

[0132] In another preferred embodiment of the present invention, theutilized method of optical data storage is capable of being used forlong-term data storage.

[0133] In another preferred embodiment of the present invention, theutilized method of data recording requires laser energy of as small as15 nJ per bit of information stored in the material.

[0134] In another preferred embodiment of the present invention, theutilized Al₂O₃:C,Mg crystalline material is substantially insensitive toroom light in both written and erased states.

[0135] Compared with known technologies for optical data storage, thepresent invention provides several advantages. Utilization offundamentally very fast electronic processes vs. comparatively slowphase change transitions and photo-induced polymerization for well knowntechniques provides a data transfer rate for one channel of up to 100Mb/s. High data storage density is achieved due to 3D capability of theproposed materials and confocal detection schemes restricted only by theblue laser light diffraction limit and NA of the optical head. Multipledata layers may be accessed in the bulk of the medium during writing andreading operations using two-photon absorption techniques and confocaldetection schemes. Non-volatile reading is achieved using one-photonexcitation of fluorescent centers causing no degradation of storedinformation. Multilevel (multivalue) data storage may further increasedata storage density due to linearity of luminescent response. Lowaverage laser light intensities required for “writing” and “reading” ofinformation (mW range) allows one to preferably use compact blue laserdiodes. Well-established and efficient crystal growth technologyproduces Al₂O₃ crystals of high optical quality.

[0136] The present invention will now be described by way of example.The example experiments described below are meant to be illustrative ofthe material and procedure described above and should not be consideredto be definitive descriptions of the invention.

EXAMPLE I

[0137] An optical data storage apparatus of the type illustrated in FIG.1 was used for demonstrating the methods of the present invention. Both“write” and “read” laser beams were produced by respective semiconductorlasers built using Nichia laser diodes. Two types of writing lasers weretested: CW modulated laser from Power Technology, producing 18 mW ofpower at 405 nm and a PicoQuant pulsed laser, generating 1.5 mW at 411nm (20 MHz, 60 ps pulse duration, 400 mW of peak power). Power of the“read” laser (440 nm, 3 mW) from Power technology was controlled usingneutral density filters. The two laser beams were directed on theAl₂O₃:C,Mg crystalline storage media through a flipping mirror, dichroicmirror and a Nikon CFI PLAN FLUOR (0.85 NA, 60X) objective lens. Thisinfinity conjugate objective lens has an optical component for manualspherical aberration compensation.

[0138] A single crystal disk of Al₂O₃:C,Mg was attached to the PolytecPI combined 3D stepper-piezo translation stage having 10 nm resolution.Selection of the focal depth or a certain data layer within 3D volume ofthe disk and correction of spherical aberrations was performed by movingthe crystal 110 in the Z-direction and by rotating the correcting collaron the objective lens. The laser-induced fluorescence was collected bythe objective lens and was reflected by the dichroic mirror through afocusing lens on the confocal pinhole. Long-pass yellow glass filterOG-515 134 was installed in front of the PMT to reject the residual bluelaser light. Fluorescence detected by the photomultiplier tube wasprocessed either by the digital oscilloscope or by the Stanford ResearchSR430 multichannel photon counter interfaced with a computer.

[0139] Recording of diffraction-limited bits at the different depth ofAl₂O₃ crystal requires careful spherical aberration compensation (SAC).SAC of the optical system was calibrated using knife-edge technique witha specially made sapphire wedge having a thickness variation from 50 to300 82 m. After the proper focal plane position and optimum SAC weredetermined, both “write” and “read” laser beam profiles were measured.The diameter of a focused laser beam at 1/e² is equal to 0.55±0.05 82 mfor a 100 to 240 μm sapphire depth range.

EXAMPLE II

[0140] An Al₂O₃:C.Mg crystal plate 1.8 mm thick was cut from a crystalboule 45 mm in diameter. It was polished on both sides and installed inthe test stand, described in the Example I, perpendicular to the opticalaxis of the objective lens with the crystal optical caxis parallel tothe polarization of the laser beam. The concentration of color centersresponsible for the blue absorption band at 435 nm and greenluminescence at 520 nm was estimated to be 17,000 centers per cubicmicron.

[0141] The test was performed in the following sequence. The “write”operation was done using either modulated CW or pulsed laser diodescontrolled by the computer interface board. Both types of lasers gavesimilar results, but the pulsed laser requires less energy per bit andproduces better spatial resolution. The crystal medium was moved in theXY plane in step increments using piezo-actuators. During readout thecrystal medium is translated in a ramp mode in X direction and with thestep-increments in Y and Z directions. The CW laser beam (15 μW, 440 nm)excited green (520 nm) fluorescence from the crystal medium.

[0142] A high density data storage process for Al₂O₃:C:Mg utilizingtwo-photon absorption during one-bit recording and confocal fluorescentdetection scheme for reading is illustrated by the image of FIG. 12. Thebit image in fluorescent contrast was obtained using the methoddescribed in the present invention and was performed using an apparatusdescribed above in the Example I and depicted in FIG. 1 in the followingsequence. The “write” operation was done with a 405 nm diode laser beamat full power and the laser pulse duration was controlled with TTLpulses from the computer interface board. Decay of the fluorescentsignal during writing operation was detected by the PMT and theoscilloscope and it was an indication of the successful writing. Duringreading operations, a CW low power blue diode laser (0.1 mW, 440 nm)modulated by another sequence of TTL pulses from a computer was used andgreen fluorescence separated by the dichroic mirror and the confocalpinhole was detected by the PMT and the photon counter.

[0143] Matrix of 3 by 3 bits spaced 5 μm apart was recorded and read asan image in fluorescent contrast (see FIG. 12). Nine bits were writtenwith 405 nm laser light and with recording energy of just 25 nJ per bit.The “read” operation was performed by scanning of the recorded area ofthe crystal storage medium with the modulated CW laser diode having awavelength at 440 nm that is longer than that of the “write beam” toprevent erasure of the information. To obtain the image of the writtenbits, scanning of the storage medium was performed with piezo-actuated3D stage from Polytec PI. The single photon pulses of the fluorescentsignal were detected using PMT and a Stanford Research SR430multichannel photon counter interfaced with a personal computer.Scanning of the crystal was performed at 0.2 μm increments and with a153 μm/s scan rate. The modulation depth of the recorded bits was about40% and a full width at half maximum for a single bit was equal to about1 μm.

EXAMPLE III

[0144] High density recording utilizing the method and the apparatusdescribed in Example I was demonstrated. A 100×100 bit image with 1 μmincrements in the X and Y directions (FIG. 13) was written using the 3Dpiezo-actuator. Each bit was written with 15 nJ of energy produced bypulsed PicoQuant diode laser (1000 pulses per bit). Reading of the bitpattern in fluorescent contrast was performed with a modulated CW-laserbeam (440 nm, 15 μW) by scanning a raster with 200 mn between lines. Animage having 500×500 pixels was obtained. Spatial profile of severalbits spaced 1 μm apart is shown in FIG. 14 and demonstrates a 12%modulation depth.

EXAMPLE IV

[0145] Single bits written using 2PA in the volume of Al₂O₃:C,Mg havedifferent dimensions in lateral and axial direction with respect to thelaser beam propagation direction. Theoretically axial size of the bitshould be 3 to 5 times bigger than lateral bit size. To determine thesize of the bits written in Al₂O₃:C,Mg in the axial direction (XZplane), bits of data were written using a step increment motion of the3D translation stage in XY plane, as described above in Example III, andthen the image of the bits in fluorescent contrast was obtained byscanning the crystal in the XZ plane (FIG. 15). In yet another test,three layers of recorded bits were obtained at an average depth of 100μm inside the crystal with a 17 μm separation between layers (FIG. 16).Each layer of bits was recorded and read with manual sphericalaberration compensation (SAC) of the objective lens, which explains somedistortion of the fluorescent image. Automated SAC should allow one toobtain up to 100 layers of data bits in the volume of the crystal having2 mm in thickness with 2D equivalent of 10 Gbit/cm² of data storagedensity or close to 1 Tbit of data per disk having surface area equal to100 cm² .

EXAMPLE V

[0146]FIGS. 17 and 18 illustrate the test of multilevel data storageutilizing the Al₂O₃:C,Mg optical storage medium. The multilevelrecording is based on the inverse proportionality between thefluorescent intensity of the written bits and the number of writingpulses. Ten bits were written in the Al₂O₃:C,Mg crystal with incrementalnumber of “writing” laser pulses. Modulation depth of the produced bitsis a nonlinear function of the number of laser pulses but neverthelesscan be digitized onto several data values and even further increase thedensity of data storage utilizing the method and the medium of thepresent invention.

EXAMPLE VI

[0147] The optical properties of Al₂O₃:C,Mg crystals utilized in thetests of the present invention now will be described. Al₂O₃:C,Mgcrystals in the shape of a boule having a 45 mm diameter were obtained.Crystals were then cut in to 1.8 mm thick disks and polished on bothsides to obtain optical quality surfaces. Optical absorption spectra ofthe Al₂O₃:C,Mg crystalline material utilized in the present inventionand of a known Al₂O₃:C crystal were obtained using Shimadzu UV-2104PCspectrophotometer and are shown in FIG. 2. The intensity of F⁺-bands at230 and 255 nm is significantly higher in Mg-doped crystals. Thatindicates higher concentration of F⁺-centers, charge compensated by theMg²⁺-ions. A blue absorption band at 435 nm indicates the creation ofaggregate F₂ ²⁺(2 Mg) defects used in the present invention. The growncrystal had 30 cm⁻¹ of absorption in the F-center band at 205 nm and anabsorption coefficient of 10 cm⁻¹ in the F⁺-centers absorption band at255 nm and 1.2 cm⁻¹ of absorption at 435 nm corresponding to absorptionof F₂ ²⁺(Mg)-center (see FIG. 2). All absorption coefficients arepresented after subtraction of the background pedestal. According toSmacula's formula, an absorption coefficient may be converted into aconcentration of F-centers equal to 8.6-10¹⁷ cm⁻³ and concentration ofF⁺-centers equal to 2.6-10¹⁷ cm⁻³ and 1.7-10¹⁶ cm⁻³ of F₂²+(Mg)-centers. The later number indicates that there are 17,000fluorescent centers per cubic micron of a storage medium.

EXAMPLE VII

[0148] To justify the appropriate wavelength range of excitation andemission light, the emission-excitation spectra of aggregate centers inAl₂O₃ doped with Mg and C in two different states were obtained (FIG. 6and FIG. 8). The spectra were obtained using a spectrofluorimeterequipped with the pulsed EG&G Xe-lamp, two scanning Acton Researchspectrographs and a cooled CCD from Princeton Instruments. It was shownthat a fresh (or erased) crystal shows an intense green luminescenceband in the region of 520 nm with the excitation band corresponding tothe blue absorption band at 435 nm (FIG. 2). After a writing operationwith 440 nm pulsed laser, the blue 435 nm absorption band (see FIG. 5)and the green emission (see FIG. 6) disappears almost completely and thecrystal shows an intensive emission band in the region of 750 nm withexcitation bands at 255 nm and 335 nm (FIG. 8) assigned to F₂ ⁺(2Mg)-centers. Both emission bands: green band at 520 and IR band at 750nm corresponding to F₂ ²⁺(2 Mg)- and F₂ ⁺(2 Mg)-centers have a shortlifetime of about 9 and 83 ns respectively (see FIGS. 4 and 7).

EXAMPLE VIII

[0149] Photo-induced transformation of color centers in Al₂O₃:C,Mgutilized for recording and erasing the data was demonstrated. Exposureof an Al₂O₃:C,Mg crystal having oxygen vacancy defects to high intensitylaser light of appropriate wavelength results in photo-inducedconversion of the same structural defect from one charged state intoanother. For example, F₂ ²⁺ (2 Mg)-centers are converted into F₂ ⁺(2Mg)-centers with 430 nm illumination (FIG. 5) and may be converted backwith 335 nm pulsed laser light. After photochromic transition induced byblue laser light, Al₂O₃:C,Mg crystals exhibit an absorption/excitationband at 335 nm (FIGS. 5 and 6), and a corresponding broad fluorescentemission at 750 nm (FIG. 6).

EXAMPLE IX

[0150] The evidence of preferred two-photon absorption in aggregateoxygen vacancy defects is provided by quadratic dependence of thephoto-ionization cross-section of these centers versus laser lightintensity (see FIG. 8). The test was performed with 415 nm, 4.5 ns laserpulses of an optical parametric oscillator illuminating a thin, 380 μm,Al₂O₃:C,Mg crystal and recording the decay constant of fluorescence as afunction of laser energy density. The photo-ionization cross-section wasthan calculated as inversely proportional to this decay constant.

[0151] Wavelength dependence of a photo-ionization cross-section of F₂²⁺(2 Mg) centers was measured as a function of wavelength. The peakposition of a photo-ionization cross-section is shifted to a shorterwavelength in comparison with the one-photon absorption/excitation band(FIGS. 3 and 9) and is another indication of the 2PA process inAl₂O₃:C,Mg crystals.

EXAMPLE X

[0152] A non-destructive readout utilizing one-photon absorption andfluorescent detection scheme was tested. A “write” operation wasperformed on Al₂O₃:C,Mg crystal using Continuum Panther OpticalParametric laser system. The laser system was tuned to generate a signalbeam at 430 nm with the pulse duration of 4.5 ns and 60 μJ/mm² of energydensity per pulse at the sample location. Reading of the written areasusing Type 1 (or so-called “negative” operation) was performed with theblue laser diode from PicoQuant (0.6 mW of average power, 60 ps pulses,and the repetition rate of 20 MHz). Fluorescence at 520 nm was detectedusing a long pass glass filter OG515, high-speed ThorLabs DET210 siliconphotodetector and Tektonix TDS-3054 oscilloscope. A fluorescent signalwith decay time of 9 ns is presented in FIG. 4 and indicates thepossibility to achieve a data transfer rate of up to 100 Mbit/s. Thefluorescent signal of unwritten area of the crystal show intensive 520nm fluorescence. The amplitude of this pulsed signal did not show anydecrease for several hours indicating that there is only one-photonabsorption during “read” operation. For comparison, area of the crystalsubjected to “writing” pulsed 435 nm laser light shows fluorescentsignal equal to only 10% of the signal from unwritten crystal.

EXAMPLE XI

[0153] Readout operation utilizing a Type 2 or “positive” fluorescentprocess was tested. The “write” operation was performed using the samecrystal sample as described in Example II and the same laser systemdescribed in Example IX. Reading of the written areas using Type 2 (or“positive” type of operation) was performed with the 335 nm UV beam fromthe same OPO laser system (100 nJ/pulse, 4.5 ns pulse duration and 10 Hzrepetition rate). Fluorescence at 750 nm was detected using a long passglass filter RG610 and a silicon photodiode DET-110 from ThorLabs, Inc.and Tektonix TDS-3054 oscilloscope. A fluorescent signal with decay timeof 80110 ns is presented in FIG. 7. An infrared fluorescence band at 750nm of a bleached (written) crystal (see FIG. 6) has a longer lifetimethan 520 nm green fluorescence of an erased crystal medium but it isstill fast enough for the data transfer rate operation of up to 10 Mb/s.

EXAMPLE XII

[0154]FIG. 5 can be used as another illustration of multilevel datastorage capabilities of the utilized optical storage medium based on theinverse proportionality between 435 and 335 nm absorption band intensityas a function of writing time using 430 nm writing beam. The Al₂O₃:C,Mgcrystal produced according to the Example I was subjected to anincrementing number of 430 nm “writing” laser pulses of the OPO lasersystem described in Example V. Each second of illumination correspondsto 10 laser pulses. Absorption at a 435 nm band associated with the 520nm fluorescent signal reduces as a function of number of writing laserpulses whereas the absorption of a 335 nm band associated with F₂ ⁺(2Mg) and infrared luminescence at 750 nm increases at the same time.

EXAMPLE XIII

[0155] A very important feature of photochromic transformations inAl₂O₃:C,Mg crystals is their high thermal stability. This high thermaland optical stability of recorded information is attributed to deeptraps created during crystal growth and is demonstrated by astep-annealing test of optical absorption bands in Al₂O₃:C,Mg crystal(FIG. 10). During recording with high intensity blue laser light, 430 nmbands convert into 335 nm and 630 nm bands [2F₂ ²⁺(2 Mg)+2hv→F₂ ⁺(2Mg)+F₂ ³⁺(2 Mg)]. Reverse transformation of optical absorption bands [F₂⁺(2 Mg)+F₂ ³⁺(2 Mg)→F₂ ²+(2 Mg)] takes place at about 650° C.

[0156] The same reverse photochromic transformation and restoration ofthe 435 nm absorption band was achieved by either pulsed 335×20 nmillumination or more efficient by sequential illumination of the crystalwith 215 nm and than with 335 nm laser light corresponding to absorptionof F and F₂ ⁺(2 Mg) centers. An original strong absorption at 435 nm andfluorescence at 520 nm was restored. The inverse photochromic processthat converts F₂ ⁺(Mg)-centers into F₂ ²⁺(Mg)-centers can be performedby two-photon absorption of laser light within 335 nm absorption band.Complete restoration of original green coloration of Al₂O₃:C,Mg can beachieved by heating the crystal to 650° C.

[0157] All documents, patents, journal articles and other materialscited in the present application are hereby incorporated by reference.

[0158] All documents, patents, journal articles and other materialscited in the present application are hereby incorporated by reference.

[0159] It is important to emphasize that the invention is not limited inits application to the detail of the particular material andtechnological steps illustrated herein. The invention is capable ofother embodiments and of being practiced or carried out in a variety ofways. It is to be understood that the phraseology and terminologyemployed herein is for the purpose of description and not of limitation.

[0160] Although the present invention has been fully described inconjunction with the preferred embodiment thereof with reference to theaccompanying drawings, it is to be understood that various changes andmodifications may be apparent to those skilled in the art. Such changesand modifications are to be understood as included within the scope ofthe present invention as defined by the appended claims, unless theydepart therefrom.

What is claimed is:
 1. A method of writing information to a data storagemedium comprising the steps of: providing a luminescent data storagemedium comprising Al₂O₃; and writing said information to saidluminescent data storage medium with an optical source.
 2. The method ofclaim 1, wherein said information is written to said luminescent datastorage medium by using a two-photon absorption technique and aphoto-ionization technique resulting in removal of an electron from acolor center in said luminescent data storage medium and moving saidelectron to a thermally stable trap in said luminescent data storagemedium.
 3. The method of claim 2, wherein said two-photon absorptiontechnique is a sequential two-step two-photon absorption technique. 4.The method of claim 2, wherein said two-photon absorption technique is asimultaneous direct two-photon absorption without intermediate levels.5. The method of claim 1, wherein said luminescent data storage mediumis written to in more than one layer at the different depths inside saiddata storage medium.
 6. The method of claim 5, wherein said opticalsource emits a laser beam from an optical read/write head and saidoptical read/write head incorporating spherical aberration compensationwith a diffraction limited depth of at least 10 microns.
 7. The methodof claim 1, wherein said luminescent data storage medium is written toat different modulation depths thereby achieving multilevel datastorage.
 8. The method of claim 1, wherein said luminescent data storagemedium comprises: a base material comprising Al₂O₃; a first dopantcomprising Mg; and a second dopant comprising carbon, wherein saidluminescent data storage medium includes a plurality of at least onetype of oxygen vacancy defect.
 9. The method of claim 8, wherein saidluminescent data storage medium includes at least one color centerhaving: an absorption in the region of 435±5 nm, an emission in theregion of 520±5 nm and a 9±3 ns fluorescence lifetime.
 10. The method ofclaim 8, wherein said luminescent data storage medium includes at leastone color center having: an absorption in the region of 335±5 nm, anemission in the region of 750±5 nm and a 80±10 ns fluorescence lifetime.11. The method of claim 8, wherein said luminescent data storage mediumincludes at least one color center having: an absorption in the regionof 435±5 nm, an emission in the region of 520±5 nm and a 9±3 nsfluorescence lifetime and at least one color center having: anabsorption in the region of 335+5 nm, an emission in the region of 750±5nm and a 80±10 ns lifetime.
 12. The method of claim 8, wherein saidluminescent data storage medium is written for a write time based on achange in fluorescence amplitude of at least 1%.
 13. The method of claim8, wherein said laser beam has a wavelength of 370 to 490 nm, inclusive.14. The method of claim 8, wherein said optical source emits a laserbeam having a wavelength of 390 nm.
 15. The method of claim 8, whereinsaid optical source emits a laser beam having a write time in the rangeof 0.1 ps to 1 ms.
 16. The method of claim 8, wherein said opticalsource emits a laser beam having has a write time of 10 ns.
 17. Themethod of claim 1, wherein at least part of the said luminescent datastorage medium is a single crystal Al₂O₃ material.
 18. The method ofclaim 1, further comprising the step of: focusing said optical source toa predetermined depth in said luminescent data storage medium.
 19. Themethod of claim 18, wherein said optical source is focused by movingsaid luminescent data storage medium with respect to said opticalsource.
 20. The method of claim 18, wherein said laser beam is focusedon said luminescent data storage medium by adjusting the position of anoptical pick-up head containing said optical source.
 21. The method ofclaim 1, further comprising the step of: moving said luminescent datastorage medium to a write position prior to said laser beam writing tosaid luminescent data storage medium.
 22. The method of claim 1, whereinsaid optical source emits a laser beam having a power density of greaterthan 10³ W/cm².
 23. The method of claim 1, wherein said optical sourceemits a laser beam having a power density of at least 10⁵ W/cm².
 24. Amethod of reading information stored on a data storage medium comprisingthe steps of: (a) exciting a luminescent data storage medium with anoptical source to thereby cause said luminescent data storage medium toemit a fluorescent light signal, wherein said luminescent data storagemedium comprises Al₂O₃ and wherein said optical source emits a readlaser beam having a wavelength in the range of an absorption band ofsaid luminescent data storage medium; and (b) measuring said laserinduced fluorescence light signal from said luminescent data storagemedium, to thereby read said information stored on said luminescent datastorage medium.
 25. The method of claim 24, wherein step (a) comprisesexciting said luminescent data storage medium using a one-photonabsorption technique without causing photo-ionization of the storagecenters to thereby cause said luminescent data storage medium to emit afluorescent light signal and thereby read said luminescent data storagemedium nondestructively.
 26. The method of claim 24, wherein step (a)comprises exciting said luminescent data storage medium using asimultaneous two-photon absorption technique without causingphoto-ionization of the storage centers to thereby cause saidluminescent data storage medium to emit a fluorescent light signal andthereby read said luminescent data storage medium non-destructively. 27.The method of claim 26, wherein said data storage medium is excited bylight from said optical source having a wavelength about two timeslonger than the wavelength of the absorption band of the saidluminescent data storage medium.
 28. The method of claim 24, whereinsaid luminescent data storage medium comprises: a base materialcomprising Al₂O₃; a first dopant comprising magnesium; and a seconddopant comprising carbon, wherein said luminescent data storage mediumincludes a plurality of at least one type of oxygen vacancy defect. 29.The method of claim 28, wherein said luminescent data storage mediumincludes at least one color center having: an absorption in the regionof 435±5 nm, an emission in the region of 520±5 nm and a 9±3 nsfluorescence lifetime.
 30. The method of claim 28, wherein saidluminescent data storage medium includes at least one color centerhaving: an absorption in the region of 335±5 nm, an emission in theregion of 750±5 nm and a 80±10 ns lifetime.
 31. The method of claim 28,wherein said read laser beam has a wavelength within an absorption bandof Al₂O₃:C,Mg centered at 335±10 nm and wherein said fluorescent lightsignal has an emission band having a wavelength range of 620-880 nm,inclusive, and being centered at 750±10 nm.
 32. The method of claim 28,wherein said fluorescent light signal is excited using light of thewavelength within an absorption band of Al₂O₃:C,Mg and centered at255±10 nm and wherein said fluorescent light signal has an emission bandhaving a wavelength range of 620 nm to 880 nm, inclusive, and beingcentered at 750±10 nm.
 33. The method of claim 28, wherein saidluminescent data storage medium includes at least one color centerhaving: an absorption in the region of 435±5 nm, an emission in theregion of 520±5 nm and a 9±3 ns fluorescence lifetime and at least onecolor center having: an absorption in the region of 335±5 nm, anemission in the region of 750+5 nm and a 80±10 ns lifetime.
 38. Themethod of claim 28, wherein fluorescent light signal has a wavelength of470 and 580 nm, inclusive, and centered at 520±10 nm.
 39. The method ofclaim 28, wherein said read laser beam illuminates said luminescent datastorage medium for the period of time between 1 ns and 10 μs.
 40. Themethod of claim 28, wherein said read laser beam illuminates saidluminescent data storage medium for about 100 ns.
 41. The method ofclaim 28, wherein said laser beam has a read time of 0.1 ps to 1 s,inclusive.
 42. The method of claim 28, wherein said laser beam has aread time of 10 ns
 43. The method of claim 28, wherein prior to step (a)said method further comprises the step of: writing to said luminescentdata storage medium with a write laser beam.
 44. The method of claim 28,wherein said read and write laser beams have a wavelength of 380 to 490nm, inclusive.
 45. The method of claim 44, wherein said read laser beamhas a wavelength longer than said write laser beam and said read laserbeam has a wavelength of about 430 to 490 nm, inclusive.
 46. The methodof claim 24, wherein information from said luminescent data storagemedium is read from more than one layer at the different depths insidesaid luminescent data storage medium.
 47. The method of claim 46,wherein step (b) comprises detecting said fluorescence signal using aconfocal detection technique.
 48. The method of claim 46, wherein saidread laser beam is emitted by said optical source disposed in aread/write head and said optical read/write head incorporates sphericalaberration compensation allowing for a diffraction limited spot at adepth of at least 10 microns.
 49. The method of claim 24, wherein priorto step (a) said method further comprises the step of: writing to saidluminescent data storage medium with a write laser beam.
 50. The methodof claim 49, wherein said read and write laser beams have the samewavelength.
 51. The method of claim 49, wherein said read and writelaser beams have different wavelengths.
 52. The method of claim 49,wherein said read and write laser beams are each focused through a lensand said lens is used for writing information to and reading informationfrom said luminescent data storage medium.
 53. The method of claim 24,further comprising the step of: moving said luminescent data storagemedium with respect to said optical source and to a read position priorto said read laser beam exciting said luminescent data storage medium.54. The method of claim 24, further comprising the step of: focusingsaid read laser beam to a predetermined depth in said luminescent datastorage medium.
 55. The method of claim 54, wherein said read laser beamis focused by moving said luminescent data storage medium with respectto said read laser beam.
 56. The method of claim 54, wherein said readlaser beam is focused by adjusting the position of an optical pick-uphead containing said optical source.
 57. The method of claim 24, whereinsaid luminescent data storage medium is read for a read time equal to aread laser beam pulse length and wherein said luminescent data storagemedium is a stationary data storage medium.
 58. The method of claim 24,wherein said luminescent data storage medium is read for a read timeequal to a ratio of a reading spot size with respect to the velocity ofsaid luminescent data storage medium and wherein said luminescent datastorage medium is a moving data storage medium.
 59. The method of claim24, wherein said read laser beam has a power density that is less thanabout 10³ W/cm².
 60. A method of erasing information stored on a datastorage medium comprising the steps of: (a) providing a luminescent datastorage medium comprising Al₂O₃, said luminescent data storage mediumhaving said information stored thereon; and (b) illuminating saidluminescent data storage medium with an optical source to thereby erasesaid information.
 61. The method of claim 60, wherein said informationis erased from said data storage medium using a two-photon absorptiontechnique.
 62. The method of claim 60, wherein said luminescent datastorage medium comprises: a base material comprising Al₂O₃; a firstdopant comprising magnesium; and a second dopant comprising carbon,wherein said luminescent data storage medium includes a plurality of atleast one type of oxygen vacancy defect.
 63. The method of claim 62,wherein said luminescent data storage medium includes at least one colorcenter having: an absorption in the region of 435±5 nm, an emission inthe region of 520±5 nm and a 9±3 ns fluorescence lifetime and at leastone color center having: an absorption in the region of 335±5 nm, anemission in the region of 750±5 nm and a 80±10 ns lifetime.
 64. Themethod of claim 63, wherein step (b) comprises illuminating of saidluminescent data storage medium with said optical source having awavelength at 335±30 nm and a power density above the threshold oftwo-photon absorption at 10³ W/cm².
 65. The method of claim 64, whereinsaid illumination is accomplished with said optical source having awavelength at 335±30 and is performed after illuminating saidluminescent data storage medium with UV light having a wavelengthcentered at 205±30 nm.
 66. The method of claim 65, wherein said UV lightis coherent.
 67. The method of claim 65, wherein said UV light isincoherent.
 68. An apparatus comprising: a luminescent data storagemedium comprising Al₂O₃; and an optical source for writing informationto said luminescent data storage medium.
 69. An apparatus comprising: aluminescent data storage medium comprising Al₂O₃; a first optical sourcefor exciting said luminescent data storage medium to thereby cause saidluminescent data storage medium to emit a fluorescent light signal wheninformation is stored on said luminescent data storage medium; andmeasuring means for measuring said emitted fluorescent light signal. 70.The apparatus of claim 69, further comprising: a second optical sourcefor writing information to said luminescent data storage medium.
 71. Theapparatus of claim 70, wherein said first and second optical sources arethe same.
 72. The apparatus of claim 71, wherein said measuring meansinclude a confocal detection means.
 73. The apparatus of claim 70,further comprising an optical head including said first optical sourceand said second optical source. 74 An apparatus comprising: aluminescent data storage medium comprising Al₂O₃; an optical source forwriting information to said luminescent data storage medium; and.compensation means for adaptive spherical aberration compensation ofsaid optical source to allow for diffraction limited spot opticaladdressing with a depth range of at least 10 microns.
 75. An apparatuscomprising: a luminescent data storage medium comprising Al₂O₃; andwriting means for writing information to said luminescent data storagemedium by using a two-photon absorption technique and a photo-ionizationtechnique resulting in removal of an electron from a color center insaid luminescent data storage medium and moving said electron to athermally stable trap in said luminescent data storage medium, saidwriting means comprising a first optical source.
 76. The apparatus ofclaim 75, further comprising: reading means for exciting saidluminescent data storage medium with an optical source having awavelength in the range of an absorption band of said luminescent datastorage medium to thereby cause said luminescent data storage medium toemit a fluorescent light signal via one-photon absorption withoutphoto-ionization of color centers in said luminescent data storagemedium, said reading means including a second optical source; and meansfor measuring said emitted fluorescent light signal.
 77. The apparatusof claim 76, wherein said first and second optical sources are the same.